Inflatable sportsballs usually comprise rubber inner bladders and elastic outer casings. The outer casing must be durable and must support internal pressurization. Typically, however, the outer casing is not substantially air impermeable, and the inner bladder provides the air barrier. Upon inflation, the outer casing is stretched tightly over the internally pressurized rubber inner bladder so that the sportsball has high resiliency and liveliness for use.
After weight, quality and design are considered, four attributes measure the usefulness of an inner bladder. These attributes are shape, room-temperature rebound, low-temperature rebound and air retention. Bladder shape influences the manufacturing processes that can be employed as well as sportsball shape retention. It is preferable for the penultimate bladder shape to be substantially identical to the inner shell of the finished sportsball. Bladder rebound correlates directly to the resiliency of a pressurized sportsball. Both room-temperature rebound and low-temperature rebound correlate to a ball's performance in play. Higher rebound values are desirous. Sportsball inner bladders tend to exhibit viscoelastic behavior, as described by time-temperature-superposition interactions of polymeric materials. Superior low-temperature rebound corresponds to better liveliness when a ball is subjected to high speed impact, for example when kicking a soccer ball or spiking a volleyball. Bladders with desirable room-temperature rebound and desirable low-temperature rebound are superior. Finally, air retention performance directly correlates to a finished ball's ability to maintain playable air pressure over long periods of time.
Commercially available bladders possess either low air permeability or good bounce and rebound capabilities, but not both superior resiliency and air retention. This is evidenced by FIFA's (Federation Internationale de Football Association) international matchball standards for soccer balls. FIFA's international standard for shape retention is less than 2% ball sphericity, which is challenging, and FIFA's standard for ball rebound is at least 115 cm at room-temperature and at least 110 cm at low-temperature, which is also challenging. However, the standard only requires that soccer balls inflated to 1 atmosphere of air pressure can lose no more than 25% air pressure after 3 days elapsed time, which is passable by nearly any conventional rubber bladder material. These FIFA limits, while strict on playability, are soft on air retention, acknowledging that the state-of-the-art has not yet achieved a bladder technology that provides for roundness, resiliency and liveliness while also providing superior air retention.
Inflatable bladders are produced by several different processes. A large number of bladders are made by latex rubber dip molding and curing (“dip molding”). Dip molding produces a variety of multiple baffled shaped balloons which can be affixed with a valve house and an inflation valve forming a finished bladder. These bladders are popular because, for example, natural rubber latex dipped bladders exhibit very good resilience, superior rebound and playability characteristics. However, they exhibit poor air retention. Also, when in the inflated but un-stretched state, these bladders form non-ideal shapes. For example, FIG. 1 shows a conventional four (4) baffled dip mold. FIG. 2 shows the polygonal, prolate shaped bladder, made using conventional dip molding processes and the dip mold of FIG. 1. This polygonal prolate shape having bladder sphericity greater than 5% is the most common latex bladder shape because many latex dip molds can be packed closely to one another in the dip molding process. Additional baffles can be designed into dip molds rendering more rounded top views, (see FIGS. 3, 4 and 5). As seen in FIG. 4, which illustrates a bladder made using an eight (8) baffled dip mold, exhibiting more than 15% bladder sphericity. With multiple baffles, these bladders are more rounded in the equatorial plane, or from the top view, but are usually pumpkin shaped or oblate shaped from the side view. (See also FIG. 5 which illustrates yet another dip mold design, (10) baffles, that renders rubber ice bag shaped moldings having bladder sphericity greater than 20%). While the ideal dip mold would comprise an infinite number of baffles to form a truly geometric spherical shape, with zero bladder sphericity However, such a dip mold design is impossible for bladder formation because the resulting balloon neck diameter would be too small to strip the balloon over the mold's large diameter without breaking or tearing. Further, as identified in FIG. 2 and FIG. 4, the baffled dip molds leave polar ribs in the form of concave grooves (20 and 40) and convex ridges (22 and 42), as artifacts of the baffled dip mold design. By their very design, conventional baffled dip molds approximate but never achieve penultimate bladders with truly round or other geometric shapes. In most cases, dip molded balloons for spherical inflated sportsball bladders are ribbed polygonal prolate or oblate spheroid shapes having bladder sphericity greater than 5%.
Using such conventional latex rubber dip molding and curing techniques, it is impossible to form a bladder having a substantially identical shape to that of the inner shell shape of a typical sportsball or gameball. In addition to the problems identified above, prior to removing the bladders from the mold, the bladders are essentially cured beyond the point at which they can be subsequently heat-set or re-molded. Because baffled dip molds are used and the bladders are cured to completion, conventional latex rubber bladders partially retain the shape and ribbed artifacts of the baffled dip mold upon inflation. For example, “spherical” bladders formed by conventional dip molding of latex rubber will produce bladders, having ridges or grooves effectively corresponding to the dip mold baffles. To date, there is no post curing or heat setting method for eliminating the partially retained shape of the baffled dip mold from conventional dip molded latex bladdrees.
Ideally, an inflated bladder should be perfectly shaped to match the shape of the intended article or sportsball. For example, a superior inner bladder for a sportsball, upon inflation, is ideally shaped substantially identically to the inner surface of the outer casing. Spherical sportsballs, like soccer balls, basketballs and volleyballs require spherical inner bladders. Oblong sport balls, like rugby and American gridiron footballs require prolate spheroid inner bladders. Pear shaped sport bags, like punching bags require pear shaped inner bladders.
Processes to manufacture bladders having desired geometric shapes are known. Most commonly, these bladders are made by a dry rubber inflation molding and curing process. This process produces a finished bladder that takes on the shape of the mold, for example, a sphere for a basketball, or a prolate spheroid shape for a rugby ball. The process uses dry millable rubber compounds such as natural rubber, EPDM, polyurethane and butyl rubber that are compounded with carbon black, sulfur, curatives and processing aids in conventional rubber mill or banbury mixer. Once fully combined, the resulting rubber or “green rubber”, being highly malleable, is calendered to the desired thickness in sheet form. The resulting green rubber sheet is folded over itself in quarter sections, and die cut to form the desired shape. For a spherical shape, the green rubber is die cut to form a cube-like balloon as illustrated in FIG. 6. A valve house (60) is adhered to one equatorial end. The green rubber bladder (71) construction is fitted with a curing inflation needle (72) and is then inserted into a split hollow heated and air pressurized mold (73) depicted in FIG. 7. While inside the heated mold, the uncured balloon is inflated so that the heated green rubber stretches and expands against the interior mold surface, thereby curing the balloon into a fully formed dry rubber inflation molded bladder. Upon a full cure, the mold is separated so that the fully cured and ideally shaped bladder (80) can be removed from the mold as illustrated in FIG. 8. The penultimate bladder from this method is a geometrical sphere with zero bladder sphericity.
Bladders made by this dry rubber inflation molding and curing process comprise a variety of natural and synthetic rubber materials, the majority of which are natural rubber or butyl rubber or laminates, mixtures, alloys or blends thereof. These bladders are known to have superior shape retention. Many synthetic rubbers and alloys of butyl rubber tend to have improved air retention but still, they tend to be comparatively deficient in rebound and playability characteristics.
Inflatable sportsballs can be produced by several different processes. A large number of high quality inflated sportsballs comprise latex dipped rubber inner bladders contained within a hand-stitched laminated synthetic casing, particularly soccer balls, rugby balls and punching bags. Many other inflated sportsballs of varying quality comprise latex dipped rubber inner bladders contained within a thermal bonded laminated synthetic casing construction, particularly soccer balls and volleyballs. Many other of the lower quality inflated sportsballs comprise latex dipped rubber inner bladders contained within a machine-stitched laminated synthetic casing. The machine stitching process has practical limits for the thickness of the laminated synthetic casing, and so, lighter weight and thinner casing materials are necessitated. In sportsballs for kicking games where heavier outer casings are preferred, like soccer, rugby and gridiron football, the quality of light-casing, machine-stitched balls is correspondingly limited. Accordingly, machine-stitched sportsballs with latex dipped bladders are not practical because they will lose their shape retention after being kicked hard a couple of times.
Many of the lower quality inflated sports balls comprise dry rubber molded rubber inner bladders contained within a machine-stitched laminated synthetic casing. For these sportsballs, there are several methods to address the inherent issues with shape retention. A very thick rubber bladder can be used. Alternatively, windings or cloth layers forming a spherical shell can be situated between the molded inner bladder and the outer casing material. In both of these cases, the inflatable sportsball's shape retention is improved. However, as described above, the rebound, moisture resistance and play characteristics of dry rubber molded bladders are deficient. The use of latex dip molded bladders is impractical because the penultimate bladders are not molded to the correct shape.
Moreover, conventional latex dip molded inner bladders are not ideal as sportsball bladders because the penultimate shape of the inflated but un-stretched inner bladders do not conform to a geometric shape or substantially shaped like the ultimate sportsball. The following table summarizes the state-of-the-art.
ConventionalConventionalBall ManufacturingLatex Dip MoldedDry Rubber InflationMethodand Cured BladdersMold Cured BladdersHand-stitched CasingsState of-the-artState of-the-artMachine-stitched CasingsNot FeasibleState of-the-artw/fiber windings(High BladderSphericity)Machine-stitched CasingsNot FeasibleState of-the-artw/floating covers(High BladderSphericityl)Polyurethane LaminateNot FeasibleState of-the-artCover w/fiber windings(High BladderSphericity)Rubber Carcass and CoverNot FeasibleState of-the-artw/fiber windings(High BladderSphericity)Thermal Bonded CasingsFeasible-NotState-of the artPreferredNot Preferred(OversizedBladders Only)
Hence, there is a need for finished latex bladders and finished articles, e.g. sportsballs, which exhibit superior rebound coupled with persistent geometrical shape retention and superior air retention. Prior to the present invention, these three attributes could not be achieved with a single rubber latex dip molded bladder shell nor could it be achieved with a single dry rubber inflation molded bladder shell.